The code sequences used during transmissions of signals with spectrum spreading are composed of a series of pulses called “chips” so as to be distinguished from the “bits” of which a data sequence is composed.
The field of the invention is that of the creation of supplementary information on a transmission of spread-spectrum signals. This may in particular prove to be useful when an already totally defined channel is available, on which it is desired to add some information, for example in the context of an emergency locating radiobeacon alert and locating system, such as Cospas/Sarsat, or within the framework of the navigation signal of a GNSS satellite. Although the issue may be applicable to a navigation signals context, it is above all of interest in a communication context.
To add information encoded on a spread-spectrum signal, the simplest solution consists in slicing the code into several code subsets and in adding binary content on portions of the sliced code by replacing the chips with their complement or otherwise. This amounts to using the techniques of TMBOC type not on the basis of a purely pilot channel to create data, but to create data on data. This approach has the defect of being deterministic (the position of the modified portion of code and the number of modified chips must be predefined in advance), and of therefore being poorly adaptable to the reception context, without establishing a user-based slicing that is very hefty in terms of costs (consumption of the band and interrogation logic). Furthermore, it requires a modification of the despreading correlators so as to be able to manage this slicing, and may therefore require considerable changing of the receivers, in particular in the most usual case where the despreading is carried out by an integrated component. Finally, this technique is not very discreet since it will be fairly simple to observe the temporal degradation of the code per chip packet, simply for example by taking an average over several successive codes, and this may be cumbersome in certain contexts.
Also known is a method called “Watermarking” in English, described for example in European patent EP 2188943 B1 (Thales) or American patent U.S. Pat. No. 8,391,490 B2 (Thales), and which consists in inserting supplementary information at fixed and known positions inside the various navigation signal spreading codes. A degradation is introduced into the reception of the signal, but the amplitude of this degradation is fixed and of known position. This solution has in particular the drawbacks of requiring a modification of the receiver so as to adapt it to the extraction of this information. This therefore limits it to post-processing functions, and it is therefore limited to processes of signal authentication by dedicated centralized reception chains, and not by the user receiver itself. Moreover, it may be rendered discernable by the repetitivity of the modified chip block.